An Automatic Transmission (AT), a Continuously Variable Transmission (CVT), or the like for an automobile is provided with a seal ring that seals the annular gap between a shaft and a housing rotating relative to each other to maintain hydraulic pressure. In recent years, an increase in fuel efficiency has been propelled as countermeasures for environmental issues, and there has been an increased demand for reducing a rotational torque in the above seal ring. In view of this, there has been known technology for providing a groove that guides sealed fluid to the side of the sliding surface of a seal ring to generate dynamic pressure (see PTL 1). In such technology, however, when sealed fluid contains foreign matter, there is a possibility that the effect of reducing a rotational torque is not sufficiently exhibited with a reduction in dynamic pressure effect and that sealing performance reduces with the acceleration of wear-out as a result of the foreign matter being put in the groove.
Note that there has been also known technology for providing a groove, through which the side of an inner peripheral surface and the side of an outer peripheral surface communicate with each other, on the side of the sliding surface of a seal ring to remove foreign matter from the sliding surface of the seal ring (see PTL 2). In such technology, however, sealed fluid leaks from a groove. Therefore, there is a possibility that a leakage amount increases.
Here, an analysis result has shown that a main factor why foreign matter is put in a groove that guides dynamic pressure as described above lies in the intrusion of the foreign matter from an abutment joint part provided in a seal ring. This point will be described with reference to FIG. 43. FIG. 43 is a partially enlarged view of the side view of a seal ring according to a conventional example. In general, a resinous seal ring 500 has an abutment joint part 510 at one portion in its circumferential direction to be easily attached to an annular groove provided on a shaft. Since a gap is formed in the abutment joint part 510, it seems that foreign matter intrudes from the gap into the sliding part between the seal ring 500 and the lateral wall surface of the annular groove and intrudes into grooves 520 that generate dynamic pressure as indicated by an arrow in the figure.